A quantitative comparison between the mHand Adapt passive adjustable hand prosthesis and its predecessor, the Delft Self-Grasping Hand

Introduction The Delft Self-Grasping Hand (SGH) is an adjustable passive hand prosthesis that relies on wrist flexion to adjust the aperture of its grasp. The mechanism requires engagement of the contralateral hand meaning that hand is not available for other tasks. A commercialised version of this prosthesis, known as the mHand Adapt, includes a new release mechanism, which avoids the need to press a release button, and changes to the hand shape. This study is the first of its kind to compare two passive adjustable hand prostheses on the basis of quantitative scoring and contralateral hand involvement. Methods 10 anatomically intact participants were asked to perform the Southampton Hand Assessment Procedure (SHAP) with the mHand. Functionality and contralateral hand involvement were recorded and compared against SGH data originating from a previous trial involving a nearly identical testing regime. Results mHand exhibited higher functionality scores and less contralateral hand interaction time, especially during release-aiding interactions. Additionally, a wider range of tasks could be completed using the mHand than the SGH. Discussion Geometric changes make the mHand more capable of manipulating smaller objects. The altered locking mechanism means some tasks can be performed without any contralateral hand involvement and a higher number of tasks do not require contralateral involvement when releasing. Some participants struggled with achieving a good initial grip due to the inability to tighten the grasp once already formed. Conclusion The mHand offers the user higher functionality scores with less contralateral hand interaction time and the ability to perform a wider range of tasks. However, there are some design trade-offs which may make it slightly harder to learn to use.

mean = 48.07± 1.36; (t(9) = 10.032,p < 0.001, d = 3.172) (S1 Fig).This learning curve seemed to last for a majority of the time however, when looking for a plateau in performance, the plateau seemed to begin at the 8 th attempt.The more limited plateau period, when compared to the previous trial's period between attempts 6 and 10 [4], leaves open the possibility that a plateau in performance was not achieved and that more learning may be achievable.S1 Fig.All mHand SHAP scores over successive attempts.Data from all participants were combined into this box plot to show scores for each successive attempt.This figure analyses the complete mHand data set.For each attempt, the line within the box represents the median, the upper and lower edges of the box represent the upper and lower quartiles respectively, and the ends of the whiskers are the maximum and minimum values.

Overall
There was a significant reduction in total contralateral interaction time between the first and last three attempts going from 5.62 ± 2.23s to 3.6 ± 1.25s (t(9) = 3.683, p = 0.005, d = 1.165) seen in S1 Table .This follows data seen during the SHAP scores section indicating participants became more proficient at using the device over time.

Grasp vs release
All participants involved the contralateral hand more during grasping (3.48 ± 1.32s) versus releasing with the mHand (1.12 ± 0.30s) (t(9) = 6.940, p < .001,d = 2.195).This might be due to the fact participants did not need to touch the device/object to release its grip.With practice, most participants reduced their contralateral hand interaction time for both grasping and releasing (S2 Fig mHand Only SHAP Scores Participants' SHAP scores increased significantly over the ten attempts at SHAP using the mHand (first three attempts: grand mean = 31.1 ± 4.01; last three attempts: grand two participants (5 and 8) saw a small increase in release interaction time between the first and last three attempts.In addition, many participants saw only limited reductions in release interaction when compared to their reductions in grasp interaction times.This might be due to a performance floor effect which is discussed in the discussion section.

Direct vs indirect
There is a general trend towards more direct (3.07 ± 1.48s) contralateral interaction compared to indirect (1.54 ± 0.34s) interaction (t( 9 S1 File.Raw Data.Spreadsheet of contralateral hand interaction data for the mHand and SHAP scores for the mHand and SGH. S2 what grip SHAP expects to be used.The number in each column under the six grip types indicates the number of times that grip was used to complete the task (note attempts participants failed are not counted).If a participant used a combination of two types of grip this was counted as 0.5 per grip type.The last row shows the sum of each grip type used.Notes about alternate methods and DNFs are noted in the 'other observations' column.(DNF = did not finish)

S2
Fig. mHand contralateral hand involvement during grasp and release interactions.Each graph represents one participant's data for grasp and release interactions over the first or last three SHAP attempts.'G' labels represent grasp interactions, 'R' labels represent release interactions.The numbers following 'G' or 'R' represent the series of attempts analysed.Whiskers represent the standard error.
) = 3.362, p = 0.008, d = 1.063).This differs from the SGH where the use of direct and indirect interactions varied widely between participants [4].With practice, this trend either continued or the amount of time spent in each levelled out (S3 Fig).Similar to the other measures, participants generally reduced their contralateral hand use for both direct and indirect interactions, 532 however, it is worth noting that two participants (2 and 6) increased their indirect interactions between the first and last three attempts.S3 Fig. mHand contralateral hand involvement during direct and indirect interactions.Each graph represents one participant's data for direct and indirect interactions over the first or last three SHAP attempts.'D' labels represent direct interactions and 'I' labels represent indirect interactions.The numbers following 'D' or 'I' represent the series of attempts analysed.Whiskers represent the standard error.

Table .
Summary of grasps used per task.